Increase conformance to Handler subclassing rules

According to documentation and existing functions:
    * _close should be close and call Handler close to properly
      unregister from existing handlers
    * emit should not re-filter logs, it is handled by the format
      function.
    * __init__ should not set a formatter, it is the responsability of
      the dev to do so. Old code forbid subclassing the formatter and
      using it.